Can online vendors change the price of a product after completion?

Can online vendors change the price of a product after completion? I thought not...but recently purchased a product within the USA with free shipping, only to have PayPal increase the price by $14.95 the next day. How can this be legal? I just got an email "[Vendor name] has made a change to your authorization" with an increased 14.95 charge. It is unclear how a vendor can authorize charges on a consumer's account.

After some research, I read that vendors can do this for shipping and tariffs, but it isn't clear how that happened for a transaction inside the US marked free shipping -- seems its on the honor system with vendors+paypal. Also, what prevents this from being a 14.95 charge only, could they then charge another $100 or another $1000?

I appealed immediately with PayPal it was rejected w/o reason and without any way to submit PDFs of the transaction history to provide evidence.

I feel really stupid for using PayPal in 2025 and obviously wont use them ever again again. Which Federal Agency would handle complaints on this, I feel completely wronged. CFPB? OCC?